Fresh air cooling [7] [1] is a rapidly spreading technique [6] to lower the PUE (Power Usage Effectiveness) of data-centres [2] by lowering the requirements for active cooling [5] with powered chillers [4]. It essentially combines using hardware capable of operating at higher temperatures with a ventilating system that relies solely or primarily on fresh air (i.e. at ambient outside temperature [9]) [8].
In these conditions, there is usually a region of the parameters space (workload, heat generation/dissipation rates . . . ) for which an isolated server will reach its critical temperature and need to idle in order to cool down to an acceptable level (resumption threshold). For a fixed job arrival rate, this will typically lead to an oscillation between the busy and idling states (and an ever-increasing queue of unprocessed jobs).
With active cooling [3] (or, more generally, without idling time), minimising delays is trivial and is best achieved by sending incoming jobs to the server with the shortest local queue (assuming they are all identical with respect to processing capability). Note that if jobs are all of the same size/duration, then the same result can be obtained by using a simple round-robin rule.
With fresh air cooling however (or, more generally, when servers may become temporarily unavailable as a direct result of their past activity) the problem becomes much more complex because the system has a memory. In summary: the “shortest queue” or “round-robin” strategy may be sub-optimal. This is intuitively understandable; for instance, it is typical for a server to have a short queue at the end of a period of intense activity. However, it will also be running very hot and potentially close to initiating a “cool down” phase, making it a poor choice for the allocation of a newly arrived job. Yet considering the temperature alone is also a bad idea: at the end of its idling period (“cool down” phase), a server will be close to its lower resumption threshold temperature, and may therefore appear as an optimal choice. However, it may still have a long queue of (partially) unprocessed jobs, which would also cause unwanted delays in the execution of the newcomer.
An object of the present invention is to provide a method to balance the workload between multiple servers so as to minimise their aggregated cooling-related idling time (and hence the delay incurred by queuing jobs) where the serves use exclusively fresh air cooling.
A further object of the present invention is to provide a method for distributing incoming jobs across a population of servers so as to maximise their aggregated processing capacity over a period of time in the absence of active refrigeration.